Wind noise is defined herein as a microphone signal generated from turbulence in an air stream flowing past microphone ports, as opposed to the sound of wind blowing past other objects such as the sound of rustling leaves as wind blows past a tree in the far field. Wind noise can be objectionable to the user and/or can mask other signals of interest. It is desirable that digital signal processing devices are configured to take steps to ameliorate the deleterious effects of wind noise upon signal quality. To do so requires a suitable means for reliably detecting wind noise when it occurs, without falsely detecting wind noise when in fact other factors are affecting the signal.
Previous approaches to wind noise detection (WND) assume that non-wind sounds are generated in the far field and thus have a similar sound pressure level (SPL) and phase at each microphone, whereas wind noise is substantially uncorrelated across microphones. However, for non-wind sounds generated in the far field, the SPL between microphones can substantially differ due to localized sound reflections, room reverberation, and/or differences in microphone coverings, obstructions, or location. Substantial SPL differences between microphones can also occur with non-wind sounds generated in the near field, such as a telephone handset held close to the microphones. Differences in microphone output signals can also arise due to differences in microphone sensitivity, i.e. mismatched microphones, which can be due to relaxed manufacturing tolerances for a given model of microphone, or the use of different models of microphone in a system.
The spacing between the microphones causes non-wind sounds to have different phase at each microphone sound inlet, unless the sound arrives from a direction where it reaches both microphones simultaneously. In directional microphone applications, the axis of the microphone array is usually pointed towards the desired sound source, which gives the worst-case time delay and hence the greatest phase difference between the microphones.
When the wavelength of a received sound is much greater than the spacing between microphones, the microphone signals are fairly well correlated and previous WND methods may not falsely detect wind at low frequencies. However, when the received sound wavelength approaches the microphone spacing, the phase difference causes the microphone signals to become less correlated and non-wind sounds can be falsely detected as wind. The greater the microphone spacing, the lower the frequency above which non-wind sounds will be falsely detected as wind, i.e. the greater the portion of the audible spectrum in which false detections will occur. Given that wind noise at hearing-aid microphones can extend from below 100 Hz to above 8000 Hz depending on hardware configuration and wind speed, it is desirable for wind noise detection to operate satisfactorily throughout much if not all of the audible spectrum, so that wind noise can be detected and suitable suppression means activated only in sub bands where wind noise is problematic. False detection may also occur due to other causes of phase differences between microphone signals, such as localized sound reflections, room reverberation, and/or differences in microphone phase response or inlet port length.
Existing approaches to WND include three techniques referred to herein as the correlation method, the difference method and the difference-sum method. These are discussed briefly below.
First, in the correlation method set out in U.S. Pat. No. 7,340,068 two microphone signals are low pass filtered (fc=1 kHz) then the cross-correlation and auto-correlation are calculated with the following equation:
                    D        =                                            ∑                              n                =                                  -                  k                                            k                        ⁢                                                  ⁢                                          x                ⁡                                  (                  n                  )                                            ⁢                              y                ⁡                                  (                                      n                    -                    l                                    )                                                                                        ∑                              n                =                                  -                  k                                            k                        ⁢                                                  ⁢                                          x                2                            ⁡                              (                                  n                  -                  l                                )                                                                        (        1        )            where x(n) and y(n) are samples of the output of microphones x and y, respectively, 1=0 for zero correlation lag, and k=0 for single-sample correlation or k>0 for correlation over a block of samples. The detector output D should theoretically approach 1 for non-wind sounds, where x(n) and y(n) should be similar, and should tend toward 0 for wind noise, where x(n) and y(n) should be dissimilar. The detector output is passed through a low-pass smoothing filter, and wind is detected when the smoothed D<0.67, and preferably when smoothed D<0.5.
Second, in the difference method for WND described in U.S. Pat. No. 6,882,736, the absolute value of the difference between two microphone signals is calculated using the equation:D=|x(n)−y(n)|  (2)where x(n) and y(n) are samples of the output of microphones x and y, respectively. The detector output, D, should theoretically approach 0 for a non-wind source, where x(n) and y(n) should be highly correlated, and increase for wind noise, where x(n) and y(n) should be less similar. The value of D is passed through a low-pass smoothing filter, and wind is detected when the smoothed value exceeds a threshold.
Third, in the difference-sum method described in U.S. Pat. No. 7,171,008, the ratio between the difference and the sum power values of two microphone signals is calculated with the equation:
                    D        =                                            ∑              n                        ⁢                                                  ⁢                                                                                                x                    ⁡                                          (                      n                      )                                                        -                                      y                    ⁡                                          (                      n                      )                                                                                                  2                                                          ∑              n                        ⁢                                                  ⁢                                                                                                x                    ⁡                                          (                      n                      )                                                        +                                      y                    ⁡                                          (                      n                      )                                                                                                  2                                                          (        3        )            where x(n) and y(n) are samples of the output of microphones x and y, respectively, over a period of time that may be one sample or a block of samples. The detector output, D, should theoretically approach 0 for a far-field source, where x(n) and y(n) should be similar, and D should tend towards 1 for wind noise, where x(n) and y(n) should be dissimilar.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
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